custom include list for source distribution patch (fixes missing checkstyle.xml)

Actually the dist plugin includes in source distribution only a predefined list of files.
When using a custom checkstyle.xml for the checkstyle report, the source distribution should include this file, elsewhere the site generation will fail complaining for the missing file.

To address the issue in a more general way I am adding a new maven.dist.src.include property which allows users to specify addictional files which should be included in source distribution.